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This package contains a simple event system with a small number of
helper classes. The listener interface is a generic interface that
can receive events. The listener bundle provides a way to register
such listeners and to create events. Different event types within
the same bundle are supported through type constants. These type
constants are interpreted as bit-fields. Thus, the bits set in 
each event MUST NOT overlap. If you want to create events, you
should use 1, 2, 4, 8, 16, 32, ... as values. A listener can be registered
for multiple events at the same time by registering the specific
bit-string. E.g., a string of 3 would register the listener for the 
events 1 and 2. The event defines constants to register a listener
for all possible event types. This is especially useful if the list
of possible events is long or the developer wants to ensure that
the listener is no longer registered for any event.
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If a developer calls the fire-event method on the listener bundle,
the ordering of event delivery is either in the order in which the
listeners have been registered or in the inverse order. An event
that is fired can contain a user-defined data object. Furthermore,
the event can be undoable. If it is undoable, the event will be
sent to all listeners until a listener sets the undo flag at the
event. If the undo flag is set, all listeners that have been 
notified so far will be notified again (together with the set
undo flag). The return type of the fire event method determines
whether the event has been requested to be undone.
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